Temperature compensation of deepwater accumulators

ABSTRACT

The method of providing an accumulator for the storage of pressurized liquids by the use of a pressurized gas, comprising providing for the storage of said pressurized gas, providing for the storage of said pressurized liquids which are pressurized by said pressurized gas, moving said accumulator to a location of lower environmental temperatures, and increasing the temperature of said pressurized gas to increase the pressure of said gas and therefore to increase the pressure of said pressurized liquid.

CROSS-REFERENCE TO RELATED APPLICATIONS

Ser. No. 10/314,361

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK

N/A

BACKGROUND OF THE INVENTION

The field of this invention is that of deepwater accumulators for thepurpose of providing a supply of pressurized working fluid for thecontrol and operation of equipment. Typical equipment includes, but isnot limited to, blowout preventers (BOP) which are used to shut off thewell bore to secure an oil or gas well from accidental discharges to theenvironment, gate valves for the control of flow of oil or gas to thesurface or to other subsea locations, or hydraulically actuatedconnectors and similar devices. The fluid to be pressurized is typicallyan oil based product or a water based product with additives to enhancelubricity and corrosion protection.

Currently accumulators come in three styles and operate on a commonprinciple. The principle is to precharge them with pressurized gas to apressure at or slightly below the anticipated minimum pressure requiredto operate equipment. Fluid can be added to the accumulator, increasingthe pressure of the pressurized gas and the fluid. The fluid introducedinto the accumulator is therefore stored at a pressure at least as highas the precharge pressure and is available for doing hydraulic work.

The accumulator styles are the bladder type having a balloon typebladder to separate the gas from the fluid, the piston type having apiston sliding up and down a seal bore to separate the fluid from thegas, and the float type with a float providing a partial separation ofthe fluid from the gas and for closing a valve when the float approachesthe bottom to prevent the escape of the charging gas.

Accumulators at the surface typically provide 3000 p.s.i. working fluidmaximum pressure. As accumulators are used in deeper water, theefficiency of conventional accumulators is decreased. In 1000 feet ofseawater the ambient pressure is approximately 465 p.s.i. For anaccumulator to provide a 3000 p.s.i. differential at 1000 ft. depth, itmust actually be precharged to 3000 p.s.i. plus 465 p.s.i. or 3465p.s.i.

At slightly over 4000 ft. water depth, the ambient pressure is almost2000 p.s.i., so the precharge would be required to be 3000 p.s.i. plus2000 p.s.i. or 5000 p.s.i. This would mean that the precharge wouldequal the working pressure of the accumulator. Any fluid introduced forstorage would cause the pressure to exceed the working pressure, so theaccumulator would be non-functional.

Another factor which makes the deepwater use of conventionalaccumulators impractical is the fact that the ambient temperaturedecreases to approximately 35 degrees F. If an accumulator is prechargedto 5000 p.s.i. at a surface temperature of 80 degrees F., approximately416 p.s.i. precharge will be lost simply because the temperature wasreduced to 35 degrees F. Additionally, the rapid discharge of fluidsfrom accumulators and the associated rapid expansion of the pressurizinggas causes a natural cooling of the gas. If an accumulator is quicklyreduced in pressure from 5000 p.s.i. to 3000 p.s.i. without chance forheat to come into the accumulator (adiabatic), the pressure wouldactually drop to 2012 p.s.i.

A fourth type accumulator has been developed which is one which ispressure compensated for depth, and is illustrated in the U.S. Pat. No.6,202,753. This style operates effectively like a summing relay to addthe nitrogen precharge pressure plus the ambient seawater pressure tothe working fluid. This means that irrespective of the seawater depth(pressure), the working fluid will always have a greater pressureavailable for work by the amount of the nitrogen precharge.

This “pressure compensated” style has numerous advantages in addition tothe pressure compensation. It allows lower gas pressures with associatedsafety, eliminates the need to recharge the system for differingoperational depths, and eliminates expensive mistakes in setting thecharge pressures.

Although the pressure compensated type has advantages over the othertypes of accumulators, it is still impacted by the change in temperatureas the seawater and therefore nitrogen becomes colder with depths.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to provide a pressure compensatedaccumulator for deepwater ocean service which compensates for thedecrease in temperature due to operating in ocean depths.

A second object of the present invention is to provide an accumulatorwhich can have the gas of the accumulator cooled at the surface to thetemperature of the subsea environment to allow realistic operationaltests at the surface without having to change the charge of theaccumulator before deploying it to ocean depths.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a partial section thru a subsea blowout preventer stackshowing applications of principles of this invention.

FIG. 2 is a half section of an accumulator of the present invention.

FIG. 3 is a partial section of the top portion of the accumulator ofthis invention.

FIG. 4 is a partial section of the accumulator of this invention showingmeans to exhaust accumulated liquids from the nitrogen chamber.

FIG. 5 is a partial section of the accumulator of this invention showingthe lower portion of the vacuum portion of the accumulator.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a blowout preventer (BOP) stack 10 is landed ona subsea wellhead system 11, which is supported above mudline 12. TheBOP stack 10 is comprised of a wellhead connector 14 which is typicallyhydraulically locked to the subsea wellhead system 11, multiple ram typeblowout preventers 15 and 16, an annular blowout preventer 17 and anupper mandrel 18. A riser connector 19, and a riser 20 to the surfaceare attached for communicating drilling fluids to and from the surface.

Blowout preventer 16 shows that an accumulator 40 of this inventionbeing connected to one of the outer cavities 41 thru line 42 and valve43. If the valve 43 is opened, fluid pressure from accumulator 40 willmove the ram 45 toward the center of the vertical bore (and seal againstan opposing ram similarly moved). Accumulator 40 can be any of the typesdescribed in the description above.

Referring now to FIG. 2, accumulator 50 has an upper plate 51, a lowerplate 52, a first cylinder 53, a second cylinder 54, a third cylinder55, a fourth cylinder 56, connecting bolts 57, connecting nuts 58, andlifting eye 59.

First cylinder 53 has an upper bore 70, a lower bore 71, a bulkhead 72,a cylinder rod 73, an upper piston 74, and a lower piston 75. Fourthcylinder 56 has an upper bore 80, a lower bore 81, a bulkhead 82, acylinder rod 83, an upper piston 84, and a lower piston 85.

Second cylinder 54 is charged with a pressurized gas, has a valveassembly 90 near the bottom, and a heating element 130. Third cylinder55 is charged with a pressurized gas and has a heating element 131.

Chambers 100, 101, 102, and 103 are pressurized with a gas such asnitrogen or helium. Chambers 115 and 116 contain a working fluidaccessible thru ports 117 and 118.

Chambers 120 and 121 contain sea water or oil at seawater pressure andthe resultant sea water pressure which comes in thru ports 122 and 123respectively. Chambers 125 and 126 contain a vacuum or may simply beallowed to have atmospheric pressure at the surface at assembly whichwill effectively be a vacuum in deep water.

Electric heating elements 130 and 131 have terminals 132, 133, 134, and135 which penetrate the upper plate 51. Electric heating elements 130and 131 are suspended within second cylinder 54 and third cylinder 55respectively. These chambers house the majority of the nitrogen gaswhich acts as the energy storage “spring” to give the accumulator apressure drive.

If an accumulator has a precharge of 3000 p.s.i. and the temperature ofthe accumulator is dropped from 84 to 34 degrees F., the pressure willdrop by 275 p.s.i. to 2725 p.s.i., giving an automatic loss ofefficiency of 29.5%.

The electricity it takes to heat one gallon of nitrogen gas 1 degree atapproximately 2000 p.s.i. is about 2.08 watt-hours. For a 100 gallonsystem to raise the temperature 50 degrees F., it will take about 2867watt-hours total. At the 480 volts in a typical deepwater drillingcontrol system, this means a total of approximately 358 amp-minutes. Ifa typical system can send 200 amps down the dual control and powercables, this means that it will take about 2 minutes to heat the gas tocompensate for the temperature differential.

This means when a substantial withdrawal occurs from the accumulatorbanks, the power umbilicals can be utilized to restore the equivalent ofthe surface temperature within a couple of minutes to give the fulloperating capacity back to the accumulators. After the operations arecompleted, the temperature will return slowly to the deepwater ambienttemperature (34 deg. F.) as the accumulators are trickle charged back totheir full capacity.

Referring now to FIG. 3, upper plate 51 has port 140 communicating thetop of first cylinder 53 with second cylinder 54, port 141 communicatingfourth cylinder 56 with second cylinder 54, and port 142 communicatingthird cylinder 55 with second cylinder 54. As the top of all fourcylinders are interconnected, the volumes of the four cylinders arecombined to provide a gas spring on the top of the two pistons 74 and84. Pistons 74 and 84 contains seals 152 and 153 respectively to sealbetween the gas chamber 100 and 103 and the working fluid chambers 115and 116.

Recesses 160 and 161 on the upper sides of pistons 74 and 84 serve tohold fluid 165 and 166. The retention of the fluid 165 and 166 in therecesses 160 and 161 serves to prevent the pressurized gas at 100 and103 from contacting and thereby tending to leak past the seals 152 and153. As liquids are characteristically easier to seal than gasses, theinsurance of liquids on both sides of the seal will improve the qualityof the sealing.

If not for the recess, as piston 74 goes to the top of the stroke ofcylinder 53, all of the liquid might be expelled thru port 140 anddumped into second cylinder 54. Likewise the liquid in the top portionof fourth cylinder 56 might be expelled thru port 141 into secondcylinder 54.

Alternately, if during the service life of the accumulator, an excessamount of liquid from chamber 115 passes by seal 152 into chamber 100,the excess amount of liquid will be expelled into the second chamber 54and excess liquids from fourth cylinder 56 will also be expelled intosecond cylinder 54.

Referring now to FIG. 4 a lower portion of second cylinder 54 is shown.When an excess amount of fluid is vented into second cylinder 54, float170 is raised pulling pin 171, link 172, and pin 173 up while pivotingup on shoulder 174. As pin 173 is pulled up valve 175 moves up and opensagainst spring 177. At this time the high gas pressure in chamber 101pushes the excess liquid out until the float 170 lowers and allows thevalve 175 to close. The excess liquid moves out through check 180 tovent out port 182 to the ocean. The check 180 will then be closed byspring 181. In this way, a single valve assembly 90 can remove anyexcess fluids which may be vented past the seals on either piston 74 or84.

Referring now to FIG. 5, a partial section of the bottom of cylinder 56is shown. In this case a check valve 190 is provided with a spring 191.If the piston 85 is simply lowered to the bottom of the stroke by thepressure of the gas from the top of the upper piston 84, a high pressurewill be generated in any liquid trapped at the bottom of the cylinder.The pressure will approximately be the sum of the pressure of theseawater entering port 122 plus the pressure of the gas in chamber 100.As the total pressure will exceed the seawater pressure (i.e. at port123), any liquids in chamber 126 will be expelled past check valve 190.

In this way, the manufacturing convenience of a four cylinderaccumulator bank is complimented with the ability to remove anycollection of liquids by a single valve assembly 90, and each of thelower vacuum chambers can be purged by a simple check valve assembly.

In addition to the ability to bring the temperature of the pressurizedgas back to the temperature at the surface, a considerable efficiencycan be obtained by increasing the temperature of the gas to elevatedtemperatures.

By selecting appropriate gasses, when the gas reaches a temperature like34 deg. F. at the bottom of the ocean, it may become a liquid andfunctionally collapse in volume, allowing the chamber of the gas tobecome smaller. This can cause a chamber of liquids to become larger, orin other words can recharge the accumulator with liquid. When the gaschamber is then reheated, the gas in the liquid state can be evaporatedand thereby repressurized. In this manner a relatively simple system forrecharging an accumulator can be located subsea.

The heating coils of this preferred embodiment illustrated can likewisebe replaced by cooling elements. The cooling elements can cool thetemperature of the gas at the surface to the temperature of the seawaterin ocean depths (typically 34 deg. F.). This will allow gas to becharged to the full pressure of 3000 p.s.i. at the surface for testingand then be normally operational at 3000 p.s.i. when it reaches subsea.

The foregoing disclosure and description of this invention areillustrative and explanatory thereof, and various changes in the size,shape, and materials as well as the details of the illustratedconstruction may be made without departing from the spirit of theinvention.

1. The method of providing an accumulator for the storage of pressurizedliquids by the use of a pressurized gas, comprising providing for thestorage of said pressurized gas, providing for the storage of saidpressurized liquids which are pressurized by said pressurized gas,moving said accumulator to a location of lower environmentaltemperatures, and increasing the temperature of said pressurized gas toincrease the pressure of said gas and therefore to increase the pressureof said pressurized liquid.
 2. The invention of claim 1, furthercomprising said pressurized gas and said pressurized liquid are inseparated chambers.
 3. The method of claim 1, further comprising usingelectricity to increase the temperature of said pressurized gas.
 4. Themethod of claim 3, further comprising using the umbilical power cable ofa subsea blowout preventer stack to provide the electric power toincrease the temperature of said gas.
 5. The method of claim 1, furthercomprising that said location of lower environmental temperatures isnear the floor of an ocean.
 6. The method of claim 1, further comprisingthat said location of lower environmental temperatures is in outerspace.
 7. The method of claim 5, further comprising that the temperatureof said pressurized gas in the subsea or outer space location isincreased to a temperature higher than the temperature of theenvironment.
 8. The method of recharging the pressure of an accumulatorwith a pressurized gas pressurizing a liquid comprising, allowing thetemperature of said gas to cool to a first temperature and liquefy andtherefore be of a smaller volume, allowing the smaller volume of saidliquefied gas to increase the volume available for said liquid andtherefore draw a greater volume of said liquid into said accumulator,and increasing the temperature of said liquefied gas to a secondtemperature to evaporate said gas, increase its pressure, and therebypressurize said liquid.
 9. The method of claim 8 further comprising saidfirst temperature is the temperature near the bottom of an ocean. 10.The method of claim 8, further comprising using electricity to increasethe temperature of said pressurized gas.
 11. The method of claim 10,further comprising using the umbilical power cable of a subsea blowoutpreventer stack to provide the electric power to increase thetemperature of said gas.
 12. The method of compensating for the loss ofgas pressure in an accumulator due to the reduction of the temperatureof the gas, comprising providing a first chamber for the storage ofpressurized liquids, providing a second chamber for the storage of saidpressurized gas, increasing the temperature of said pressurized gas toincrease the pressure of said gas and therefore to increase the pressureof said pressurized liquid.
 13. The method of claim 12, furthercomprising using electricity to increase the temperature of saidpressurized gas.
 14. The method of claim 13, further comprising usingthe umbilical power cable of a subsea blowout preventer stack to providethe electric power to increase the temperature of said gas.
 15. Themethod of claim 12, further comprising that the method for increasingthe temperature of said pressurized gas is by using heating elementswithin said second chamber.
 16. The method of claim 12, furthercomprising that the method for increasing the temperature of saidpressurized gas is by using heating elements outside of said secondchamber.
 17. The method of providing an accumulator for the storage ofpressurized liquids by the use of a pressurized gas, comprisingproviding for the storage of said pressurized gas in a first chamber,providing for the storage of said pressurized liquids which arepressurized by said pressurized gas, reducing the temperature of saidgas at the surface to the anticipated temperature of the deepwaterenvironment, setting the pressure of the gas at the surface to thedesired subsea pressure, lowering the accumulators to the subseaenvironment, and allowing the accumulators to become the sametemperature as the subsea environment.
 18. The method of claim 17,further comprising that the reducing of the temperature of said gas atthe surface is accomplished by using cooling coils within said firstchamber.
 19. The method of claim 17, further comprising that thereducing of the temperature of said gas at the surface is accomplishedby using cooling coils external to said first chamber.